This disclosure relates to gas turbine engines. In particular, this disclosure relates to repaired turbine exhaust strut heat shield vanes and repair methods.
An industrial gas turbine generally includes a stationary exhaust duct through which hot combustion gases are flowed before exiting the main engine into a power turbine used to, for example, generate electricity. The exhaust duct commonly includes an inner annular ring forming the inner wall of the gas path and an outer annular ring forming the outer wall of the gas path. Radially extending struts circumferentially distributed about the engine longitudinal axis span the gas path between and connect to the inner and outer annular rings. Each radially extending strut may be surrounded by an airfoil, thereby forming a stationary vane. The vane acts both to direct gas flow through the exhaust duct and to shield the load bearing struts from heat.
Hot combustion gases discharging from the turbine into the exhaust duct during engine operation commonly have a residual velocity component in the tangential direction with respect to the gas path. The tangential velocity component of the hot combustion gases is undesirable as it detracts from the useful energy that may be extracted from the gas flow. Converting the tangential velocity to axial velocity increases the axial thrust produced by the engine, which in turn increases engine performance. The tangential velocity component of the gas flow may be redirected axially by the vanes surrounding the struts of the exhaust duct. More specifically, each vane airfoil may be contoured to aerodynamically redirect the flow of gases from a tangential direction to an axial direction.
The durability of the turbine exhaust case and its associated load bearing struts depends largely on the material selected for the case. It is generally desirable to select the material of the case struts based on structural versus, for example, thermal properties. Therefore, it is not uncommon to shield the struts by encapsulating them with non-structural vanes that act as heat shields. In this way the exhaust case struts may be formed of a material that exhibits optimal structural properties, but may not be able to withstand the operating temperatures of the engine without the shielding of the vanes. The vanes, on the other hand, may be formed of a material that exhibits optimal thermal, and oxidation and corrosion resistance properties, while not necessarily exhibiting high strengths.
The turbine exhaust case strut heat shield vanes may experience a loss of material due to a combination of abrasion, exfoliation, oxidation, hot corrosion and some mode of metallurgical attack during engine operation. The material on the vanes may have temperature limits hundreds of degrees above the engine operating temperatures. Although the damage observed on the vanes may not have structural repercussions, the damage may compromise, to an increasing degree over time, the heat shielding function of the vanes. As a result, structural exhaust case components, i.e. the struts, which are intended to be protected by the heat shield vanes may also be compromised. For example, there is an increasing risk that the strength properties of the struts will be degraded and that the struts may yield during operation.